An engine gasket referred to as a head gasket is a sealing member which is mounted between a cylinder head and a cylinder block and which prevents leakage of combustion gas or engine cooling water or oil.
In the past, as a head gasket, a composite type gasket having a structure in which a compressive member was wrapped in mild steel was used, but at present, almost all are metal gaskets essentially comprising a metal sheet.
A metal gasket for an engine (a head gasket) has the same outline as the portion to be sealed with the gasket and is constructed from about three sheets of stainless steel having circular holes corresponding to combustion chambers (cylinders) stacked on top each other. An annular projection referred to as a bead is formed around each hole in the gasket [see FIGS. 3(a) and (b)], and sealing with respect to a high-pressure combustion gas or the like is guaranteed by intimate contact resulting from the resilience of the bead. The entire surface of the gasket on the outer side of the bead is thinly coated with rubber in order to prevent the formation of scars on the surface of the steel sheets and to prevent the leakage of cooling water, oil, and the like running along the gasket. When forming the coating of rubber, heat treatment is typically carried out at a temperature up to about 350 ° C. for a few minutes.
In the past, SUS 301 and SUS 304, which are metastable austenitic stainless steels, were widely used in metal gaskets for engines. These materials are normally used after cold rolling (temper rolling) performed for the purpose of strength adjustment. Due to work hardening accompanying strain induced martensitic transformation, a high strength is obtained relatively easily. In addition, due to the hardening caused by strain induced martensitic transformation in deformed portions, the so-called TRIP effect in which the material is uniformly deformed with suppressed local deformation is obtained, so these steels are distinguished among various stainless steels as having excellent workability.
However, even with these materials, as is the case with other metal materials, a decrease in workability accompanying an increase in strength is unavoidable. With these materials, it is difficult to both satisfy an even higher strength which is demanded with an increase in the output of recent engines and a sufficient level of workability to form complicated shapes which are desired as weights decrease, i.e., as sizes decrease.
The above-described stainless steels, if they are in the form of a flat sheet, as their strength increases, their fatigue strength also increases. However, when they are used to form conventional metal gaskets for engines, as the shape of the gaskets becomes more complicated, it was observed that defects such as cracks (minute cracks in the surface of the steel sheet), wrinkles, and the like occurred at the time of bead formation due to insufficient workability of the steel material, thereby causing a significant decrease in fatigue properties after working.
Therefore, there have been many proposals of methods in which working (such as by punching and bead formation) of a stainless steel sheet into a gasket is carried out in a state in which necessary workability can be guaranteed (before strengthening), and then heat treatment is carried out to achieve age hardening in order to increase strength.
Specifically, a material which uses a steel corresponding to the above-mentioned SUS 301 or SUS 304 and which is increased with respect to resistance to elastic deformation (spring properties) such as Young's modulus and proportional limit of spring by strain aging and a manufacturing method therefor are proposed in JP P03-68930B and P07-65110B. A high strength material having increased hardness and strength (tensile strength) by the addition of a precipitation strengthening element such as Si, Mo, Cu, or Ti and a method for its manufacture are disclosed in JP P04-214841A and P05-117813A.
In addition, the use of a precipitation strengthening type stainless steel such as SUS 630 or SUS 631 which achieves high strength primarily by precipitation strengthening has also been proposed.
However, while strain aging improves spring properties and increases the resiliency of a bead, the increase in hardness and strength is small. Therefore, when a gasket is mounted between a cylinder head and a cylinder block and clamped by bolts or the like, there was the problem that permanent set in which the bead was crushed and its height decreased took place.
On the other hand, precipitation strengthening typically requires heat treatment for a long period at a relatively high temperature of 400-600° C. Since a rubber coating cannot withstand such a high temperature, heat treatment for precipitation strengthening must be carried out after working of the gasket and before rubber coating. It is a heavy burden for gasket manufacturers to perform heat treatment at such a high temperature, and due to addition of the step of heat treatment for precipitation strengthening, the process of manufacturing a gasket becomes complicated. Therefore, in the past, it was difficult to make practical use of a metal gasket having an increased strength by use of precipitation strengthening. Another problem of the heat treatment performed at a high temperature for a long period for the purpose of precipitation strengthening is that it tends to cause the formation of coarse precipitates, which become a starting point from which fatigue fracture originates.
An object of this invention is to provide a high performance metal gasket which can be advantageously manufactured industrially and which has high strength and good fatigue properties so as to enable it to be utilized in recent high performance engines, as well as a method for its manufacture.
Another object of this invention is to provide a stainless steel for a metal gasket which has excellent workability at the time of working to form into a gasket and which undergoes precipitation strengthening by heat treatment at a temperature of around 300° C. (200-350 ° C.) which is performed at the time of rubber coating so that it can be used to manufacture the above-described high performance metal gasket without performing additional heat treatment for precipitation strengthening, as well as a method for its production.